The present invention relates to apparatus and methods for processing semiconductor wafers or other semiconductor workpieces, in a chamber containing a plasma created to ionize a deposition material or etch a workpiece.
Certain semiconductor device fabrication procedures include processes in which a material is sputtered from a target onto a workpiece, such as a semiconductor wafer or a group of semiconductor wafers, in the presence of a plasma that ionizes the sputtered material. Material is sputtered from the target, which is appropriately biased, by the impact of ions created in the vicinity of the target.
The workpiece is mounted on a support and may be biased to a potential selected to attract the sputtered, ionized material to the workpiece. Typically, the sputtered material once ionized, is composed of positive ions and the workpiece is negatively biased.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil which is located within the chamber induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
As described in copending application Ser. No. 08/680,335, entitled "Coils for Generating a Plasma and for Sputtering," filed Jul. 10, 1996 (Attorney Docket # 1390CIP/PVD/DV) and assigned to the assignee of the present application, which application is incorporated herein by reference in its entirety, it has been recognized that the coil itself may provide a source of sputtered material to supplement the deposition material sputtered from the primary target of the chamber. Application of an RF signal to a coil coupled to a blocking capacitor can cause the coil to develop a negative bias which will attract positive ions that can impact the coil, causing material to be sputtered from the coil. Because the material sputtered from the coil tends to deposit more thickly at the periphery of the workpiece area, the center thick tendency for material sputtered from the primary target can be compensated by the edge thick tendency for material sputtered from the coil As a result, uniformity can be improved.
One problem encountered in the operation of apparatus of the type described above is that variations occur in the manner in which the sputtered material is deposited on the workpiece during operation of the apparatus over a period immediately following startup of the apparatus. As a result, processing of the first several workpieces may not be satisfactory, even though sputtering material has been consumed, as has a portion of the operating life of the apparatus.